The present invention relates to microelectromechanical switch and relay structures, and more particularly to overdrive structures to be used in conjunction with electrostatically activated switch and relay structures.
Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices. One such device which leverages thermal expansion to move a microdevice is found in U.S. Pat. No. 5,475,318, entitled xe2x80x9cMicroprobexe2x80x9d, issued on Dec. 12, 1995, in the name of inventors Marcus et. al. In that device a micro cantilever is constructed from materials having different thermal coefficients of expansion; When heated, the bimorph layers arch differently, causing the micro cantilever to move accordingly. A similar mechanism is used to activate a micromachined thermal switch as described in U.S. Pat. No. 5,463,233, entitled xe2x80x9cMicromachined Thermal Switchxe2x80x9d, issued on Oct. 31, 1995, in the name of inventor Norling.
Electrostatic forces have also been used to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or Mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,339, entitled xe2x80x9cMethod for Making Rolling Electrode for Electrostatic Devicexe2x80x9d, issued on May 12, 1981, in the name of inventor Kalt. These devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
Micromachined MEMS electrostatic devices have been created which use electrostatic forces to operate electrical switches and relays. Various MEMS relays and switches have been developed which use relatively rigid cantilever members separated from the underlying substrate in order to make and break electrical connections. Typically, contacts at the free end of the cantilever within these MEMS devices move as the cantilever deflects, so that electrical connections may be selectively established. As such, when the contacts are connected in these MEMS devices, most of the cantilever remains separated from the underlying substrate. For instance, U.S. Pat. No. 5,367,136, entitled xe2x80x9cNon-Contact Two Position Microelectronic Cantilever Switchxe2x80x9d, issued on Nov. 22, 1994, in the name of inventor Buck; U.S. Pat. No. 5,544,001, entitled xe2x80x9cElectrostatic Relayxe2x80x9d, issued on Aug. 6, 1996, in the name of inventors to Ichiya, et al., and U.S. Pat. No. 5,278,368, entitled xe2x80x9cElectrostatic Relayxe2x80x9d, issued Jan. 11, 1994, in the name of inventors Kasano, et al. are representative of this class of microengineered switch and relay devices.
Another class of micromachined MEMS switch and relay devices include curved cantilever-like members for establishing electrical connections. For instance, U.S. Pat. No. 5,673,785, entitled xe2x80x9cMicromechanical Relayxe2x80x9d, issued on Oct. 7, 1997, in the name of inventors Schlaak, et al., describe a microcantilever that curls as it separates from the fixed end of the cantilever and then generally straightens. The electrical contact is disposed at the generally straight free end of the microcantilever. When electrostatically attracted to a substrate electrode, the Schlaak devices conform substantially to the substrate surface except where the respective electrical contacts interconnect. In addition, a technical publication by Ignaz Schiele et al., titled Surface-Micromachined Electrostatic Microrelay, 1198, Sensors and Actuators, also describes micromachined electrostatic relays having a curled cantilever member. The Schiele cantilever initially extends parallel to the underlying substrate as it separates from the fixed end before curling away from the substrate. While the cantilever member having a contact comprises a multilayer composite, flexible polymer films are not used therein. As such, the Schiele devices do not describe having the cantilever member conform substantially to the underlying substrate in response to electrostatic actuation thereof.
MEMS electrostatic switches and relays are used advantageously in various applications because of their extremely small size. Electrostatic forces due to the electric field between electrical charges can generate relatively large forces given the small electrode separations inherent in MEMS devices. However, problems may arise when these miniaturized devices are used in high voltage applications. Because MEMS devices include structures separated by micron scale dimensions, high voltages can create electrical arcing and other related problems. In effect, the close proximity of contacts within MEMS relays and switches multiplies the severity of these high voltage problems. Further, relatively high electrostatic voltages are required to switch high voltages. The air gap separation between the substrate electrode and moveable cantilever electrode affects the electrostatic voltage required to move the cantilever electrode and operate the switch or relay. A relatively large air gap is beneficial for minimizing high voltage problems. However, the larger the air gap, the higher the voltage required to operate the electrostatic switch or relay. As such, traditional MEMS electrostatic switch and relay devices are not well suited for high voltage switching applications.
Recent innovations have led to MEMS switches and relays that leverage the benefits of electrostatic forces and provide for devices capable of switching high voltages with relatively low electrostatic voltages. Additionally, these devices have shown to be instrumental in overcoming at least some of the arcing and high voltage operational problems. See for example, U.S. patent application Ser. No. 09/345,722, entitled xe2x80x9cHigh Voltage Micromachined Electrostatic Switchxe2x80x9d, filed on Jun. 30, 1999, in the name of inventor Goodwin-Johansson and assigned to the same assignee, MCNC, as the present invention. That application is expressly incorporated by reference as if fully setforth herein. A key attribute to the structures discussed in the aforementioned application is the availability of large electrostatic forces due to the flexible metallized polymer film coming into direct contact with the substrate that contains the stationary electrode.
In the switches and relays discussed in the Goodwin-Johansson ""722 Application the switch contacts that are disposed in the substrate are typically designed as posts that extend slightly above the surface of the substrate structure. The release layer operation employed during switch fabrication generally results in the posts having a flat plan view topography. As such, the majority of the area of the contact in the flexible composite is generally the same spacing from the substrate contacts as the rest of the flexible composite is from the substrate. As a result of this equal spacing, when the switch closes (i.e. the entirety of the flexible composite lies generally parallel with the substrate) minimal contact force results between the substrate contacts and the flexible composite contacts. This lack of overdrive capability can result in unacceptable contact resistances. What is needed are contact structures within the MEMS electrostatic switching and relay devices that are capable of imparting overdrive capabilities and insuring sufficient contact force between the substrate contacts and the flexible composite contacts.
The present invention provides improved MEMS electrostatic switch and relay devices that can provide overdrive potential to the contacts by adding surface topography to the mating surfaces of the contacts. Further, a method for using the MEMS electrostatic switch and/or relay device according to the present invention is provided.
A MEMS device driven by electrostatic forces according to the present invention comprises a substrate, at least one substrate electrode, at least one substrate contact, a flexible composite, at least one flexible composite contact, and an insulator. A substrate defines a planar surface upon which the MEMS device is constructed. The substrate electrode(s) typically forms a layer on the surface of the substrate. The flexible composite generally overlies the substrate electrode(s). In cross section, the flexible composite comprises an electrode element layer and at least one biasing element layer. The flexible composite across its length comprises a fixed portion attached to the underlying substrate, and a distal portion moveable with respect to the substrate electrode. The composite contact is attached to the composite. In addition, an insulator electrically isolates and separates the substrate electrode from the electrode layer of the flexible composite. Applying a voltage between the substrate electrode and flexible composite electrode creates an electrostatic force that attracts the moveable distal portion of the composite to the underlying substrate. As such, the substrate contact and composite contact are electrically connected together in response to the application of electrostatic force.
In one embodiment of the invention the at least one substrate contact will define protrusions on the contact surface that extend toward the at least one flexible composite contact. The protrusions on the surface of the contacts add topography to the contacts and provide for overdrive potential when the contacts are brought into contact. The protrusions allow for greater contact force over a larger surface area between the contacts, and lower contact resistance.
In another embodiment of the invention the at least one flexible composite contact will define protrusions on the contact surface that extend toward the at least one substrate contact. Similarly, the protrusions on the surface of the contacts add topography to the contacts and provide for overdrive potential when the contacts are brought into contact. The protrusions allow for greater contact force over a larger surface area between the contacts, and lower contact resistance.
One embodiment of the MEMS electrostatic device according to the present invention forms the electrode element and biasing element of the flexible composite from one or more generally flexible materials. Layers comprising the composite can be selected such that the flexible composite substantially conforms to the surface of the substrate when the distal portion of the flexible composite is attracted to the substrate. In addition, layers comprising the composite can be selected such that the distal portion can be positionally biased with respect to the substrate when no electrostatic force is applied. Other embodiments define the relative positions of the substrate contact and the substrate surface, as well as the characteristics of the surface of the substrate contact. One embodiment provides a plurality of substrate contacts, which optionally may be interconnected in series or in parallel. The position of the insulator relative to the substrate electrode, substrate contact, and substrate is further defined in one embodiment. One embodiment describes the characteristics of the electrode layer and biasing layers comprising the flexible composite.
In a further embodiment, the characteristics of the distal portion of the flexible composite are described. One embodiment describes the attributes of, and positions of, the composite contact relative to the flexible composite. Further, in one embodiment, the composite contact comprises a plurality of contacts, which optionally may be connected in series or in parallel. An embodiment also details the shapes and relative sizes of the substrate electrode and composite electrode. Other embodiments further comprise a source of electrical energy and electrically connected to at least one of the substrate contact and the composite contact, or electrically connected to at least one of the substrate electrode and the composite electrode. Optionally, these embodiments may further include a diode or a switching device.
In addition, another embodiment of the present invention provides a method of using the electrostatic MEMS devices described above. The method comprises the step of electrically isolating at least one of the substrate contact or the composite contact from its respective associated substrate electrode or composite electrode. The method comprises the step of selectively generating an electrostatic force between the substrate electrode and the electrode layer of the flexible composite, and moving the flexible composite toward the substrate. Lastly, the method comprises the step of electrically connecting substrate contact and the flexible composite contact in a circuit and overdriving the flexible composite contact or substrate contact into the corresponding substrate contact or flexible composite contact so as to minimize contact resistance.